The Solar Wind's Dangerous Acceleration

Just outside Earth's atmosphere, extending into space for thousands of kilometers, lurk the Van Allen radiation belts. Normally, the ionized electrons there pose a manageable danger to satellites and spacecraft. But every month or so, radiation levels can suddenly spike, generating extremely energetic streams of electrons that fry electronic circuits or damage the DNA of astronauts unlucky enough to be there.

Because of the risks, physicists have been attempting for decades--nearly since the Van Allen belts themselves were discovered in 1958--to find what triggers the outbursts. And outbursts they are, producing thousands of times more energy than anything generated by the solar wind. A team at Los Alamos National Laboratory in New Mexico has examined data collected over the past few years from the laboratory's instruments riding aboard five satellites orbiting Earth at various altitudes. In a letter published online 1 July in Nature Physics, they have fingered the culprit: a still-undetermined acceleration process inside the Van Allen belts.

When they analyzed all of the satellite data, the three researchers found, as expected, that the peak periods of high-energy electrons in the belts correlated very well with the incidence of solar storms. These storms can stream swarms of particles toward Earth, but at lower energies than the electrons in question. But the data also showed that the energy of the particles increased very quickly near the center of the Van Allen belts without the influence of an outside source--something that had not been demonstrated before. "We were trying to assemble a snapshot of the missing data pieces," says space physicist and co-author Reiner Friedel. And using the array of satellites "allowed us to see things that we couldn't see with a single satellite." Co-author Yue Chen is certain that the particles get most of their boost from interactions with the electromagnetic waves. The team is now developing new computer simulations of the acceleration process.

Using the data from multiple satellites represents "a simple and elegant achievement," says space scientist Larry Paxton of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. The Los Alamos team showed very clearly that the high-energy electrons are created locally by acceleration within the Van Allen belts, he says. The findings should help improve the design and operation of NASA's Radiation Belt Storm Probe mission, slated for launch in 2012, Paxton says.